The arginine-glycine-aspartic acid (Arg-Gly-Asp; RGD) motif of extracellular matrix (ECM) components such as fibronectin (Pierschbacher and Ruoslahti, 1984) and vitronectin binds to integrins (Ruoslahti and Pierschbacher, 1987; D'Souza S E et al., 1991; Joshi et al, 1993; Koivunen et al., 1994). Integrin-mediated adhesion leads to intracellular signaling events that regulate cell survival, proliferation and migration. About 25 integrins are known, and at least eight of them bind the RGD motif as the primary recognition sequence in their ligands.
Data obtained by phage display methods (Pasqualini and Ruoslahti, 1996) screening for RGD-containing peptides have shown their selective binding to endothelial lining of tumor blood vessels (Ruoslahti, 1996; Pasqualini et al., 1997).
Because the expression of integrins is reported to be high on activated, but more restricted on quiescent, endothelial cells (ECs), small synthetic RGD-containing peptides have been proposed as antagonists impairing the growth of vascular endothelial and tumor cells. RGD peptides also retard signal transmission, affect cell migration and induce tumor cell regression or apoptosis (Su et al., 2002). RGD-analogues are used in tumor imaging (Haubner et al., 2001), anti-angiogenesis approaches (Kawaguchi et al., 2001; Pasqualini et al., 2000), and in tumor targeting of radionucleotides (van Hagen et al., 2000) and chemotherapeutic drugs (Arap et al., 1998; Zitzmann et al., 2002).
Integrins are also expressed on cancer cells and play an important role in the invasion, metastasis, proliferation and apoptosis of cancer cells. Metastatic invasion of tumor cells into preferred organs may represent cell-homing phenomena that depend on the adhesive interaction between the tumor cells and organ-specific endothelial markers (Ruoslahti and Rajotte, 2000). By binding to integrin of either endothelial or tumor cells, RGD peptides are capable of modulating in vivo cell traffic by inhibition of tumor cell-ECM and tumor cell-EC attachments, which are obligatory for metastatic processes. Several studies have indicated that RGD-containing compounds can interfere with tumor cell metastatic processes in vitro (Goligorsky et al., 1998; Romanov and Goligorsky 1999) and in vivo (Saiki et al., 1989; Hardan et al., 1993).
Peptides that are specific for individual integrins are of considerable interest and of possible medical significance. The αvβ3 integrin was the first integrin shown to be associated with tumor angiogenesis. RGD peptides that specifically block the αvβ3 integrin show promise as inhibitors of tumor and retinal angiogenesis, of osteoporosis and in targeting drugs to tumor vasculature (Assa-Munt et al., 2001). Coupling of the anticancer drug doxorubicin or a pro-apoptotic peptide to an αvβ3 integrin-binding RGD peptide yields compounds that are more active and less toxic than unmodified drugs when tested against xenograft tumors in mice (Ruoslahti, 2000; Arap et al., 1998; Arap et al., 2002; Ellerby et al., 1999). Consequently, a great amount of work was invested in designing and producing integrin-binding peptides and peptidomimetics (Haubner et al., 1996; Locardi et al., 1999; Lark et al., 1999; Raboisson et al., 2006; Belvisi et al., 2005; Dijkgraaf et al., 2006; Banfi et al., 2007; U.S. Pat. No. 5,849,692).
U.S. Pat. No. 6,576,239, EP 0927045 and WO 98/010795 disclose a conjugate comprising a tumor horning peptide comprising the amino acid sequence RGD or NGR, said peptide linked to a therapeutic or diagnostic moiety, provided said moiety is not a phage particle. The therapeutic moiety may be a cytotoxic agent or a cancer chemotherapeutic agent such as doxorubicin. The conjugate selectively homes to angiogenic vasculature upon in vivo administration. The tumor homing peptide may be a linear or cyclic peptide of up to 20 or 30 amino acids or of 50-100 amino acids in length. One preferred peptide is the cyclic nonapeptide CDCRGDCFC or H-Cys*-Asp-Cys*-Arg-Gly-Asp-Cys*-Phe-Cys*-NH2.
WO 2008/023378 discloses a conjugate of an RGD-containing peptide or an RGD peptidomimetic and a photosensitizer selected from a porphyrin, a chlorophyll or a bacteriochlorophyll.